Implantable device with digital waveform telemetry

ABSTRACT

A technique for acquiring and accessing information from a medical implantable device is provided. Analog waveforms of interest are sensed and processed by signal acquisition circuitry. Analog parameters of interest are applied to selector switches which are controlled by a logic circuit. The logic circuit is also coupled an A/D converter for converting the analog signals to digital values. The digital values are stored in dedicated registers and are available for telemetry to an external device upon receipt of a request or prompt signal. When a digitized value is accessed and telemetered, the control logic circuit changes the conductive state of the selector switches to apply the corresponding analog signal to the A/D converter. The resulting digital value is applied to the corresponding register to refresh the accessed and telemetered value. The technique permits the external device to request and configure the implanted device to send only digitized values of interest. The technique also makes efficient use of the A/D converter, which consumes energy only as needed to refresh the memory when digital values are accessed and telemetered.

CLAIM OF PRIORITY

This application is a Continuation of application Ser. No. 09/020,278filed on Feb. 6, 1998, now issued as U.S. Pat. No. 5,978,713.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to implantable devices, such as cardiacstimulators, designed to be situated within a living body and toexchange information with devices located outside the body. Moreparticularly, the invention relates to a novel technique for processingand exchanging data between an implantable device and a remote devicewhich makes efficient use of power and signal processing capabilitieswithin the implantable device.

2. Description of the Related Art

In recent years increasingly sophisticated systems have been developedfor monitoring and controlling certain physiological processes viaimplanted devices. Such devices are typically placed within a patient'sbody and remain resident within the patient's body over extended periodsof time. One such device, commonly referred to as a cardiac stimulator,is commonly implanted in a patient's chest region and includes circuitryboth for monitoring the functioning of the patient's heart as well asfor providing stimulus for the heart when needed.

Conventional implantable cardiac stimulators include one or moreelectrical leads which extend between electronic circuitry providedwithin the device housing and portions of the patient's heart. Forexample, leads extending from the stimulator may be terminated in theright atrium and right ventricle of the patient's heart to provide bothsensing and stimulation capabilities. The circuitry is programmed toexecute desired functions, such as monitoring, stimulating, and storingof diagnostic or other data. A power supply is implanted with the deviceto furnish the electrical energy needed for its operation.

Through their relatively short history, cardiac stimulators and otherimplantable devices have experienced very considerable evolution. Forexample, early cardiac stimulators provided fixed rate stimulatingpulses designed to regulate the patient's heart beat only. Laterdesigns, sometimes referred to as “demand” pacemakers, also offeredheart monitoring capabilities, providing stimulating pulses only asneeded based upon the monitored functioning of the heart. Furtherimprovements in cardiac stimulators included programmable ratepacemakers, dual chamber pacemakers, and “rate-responsive” pacemakers,each providing increased flexibility and adaptability of the monitoringand stimulation functions to more closely conform to the needs andphysiological parameters of the patient, such as the patient's level ofphysical activity.

Throughout the evolution of cardiac stimulators and other implantabledevices, a persistent problem has been the efficient provision and useof electrical energy. In general, the power source, typically includinga specially designed electric battery, is implanted with the electroniccircuitry to provide all power necessary for the monitoring,stimulation, programming and other functions of the implantable deviceover extended periods of time, often measured in years or decades. Toprovide the longest possible life to the implanted power source,therefore, it is generally a goal in the design of such devices toreduce the power needed for all aspects of their function. For example,the replacement of early fixed rate pacemakers with demand pacemakerssignificantly reduced the energy continuously dispensed by the device bygenerating stimulating signals only as needed, thereby prolonging theeffective life of the power source. Other developments have alsoextended the useful life of such power sources, although furtherimprovements are still needed.

A particularly useful function of implantable devices involves theability to transmit and to receive information between the implantabledevice and an outside programming or monitoring unit. Data exchangebetween the implantable device and the external unit permits parameters,such as physiological data, operational data, diagnostic data, and soforth, to be transmitted from the implantable device to a receiver fromwhich the data can be accessed and further processed for use by anattending physician. The data is particularly useful for gaining insightinto the operation of the implantable device as well as the state of thepatient's organs and tissues. The ability to exchange data in thismanner also permits the physician to reprogram or reconfigure theimplantable device as may be required from time to time due to evolutionof the patient's condition.

Data exchange between an implantable device and a remote, outside deviceis often accomplished by “waveform telemetry” in which the data isconveyed through the patient's tissue and skin. Early waveform telemetrysystems employed in implantable cardiac stimulators transmitted signalsthrough analog encoding. For example, in one known type of pacemaker,analog samples representing operational or physiological parameters aretransmitted as the pulse position of a radio-frequency pulse train. Thepulse train is output by either the implantable device or the outsidedevice, and is interpreted or decoded upon receipt by the other device.While such techniques are extremely useful for gaining access toinformation relating the performance of the patient's organs and of theimplantable device, analog telemetry circuits typically yield lowresolution and often AC-coupled and uncalibrated signals, effectivelylimiting their utility and reliability.

To address the shortcomings of analog telemetry systems, digitaltelemetry schemes have been developed. For example, certain digitaltelemetry systems are presently in use wherein a radio-frequency carrieror radio-frequency pulse train is modulated by digital informationcorresponding to samples of the analog signals to be telemetered. Suchdigital data communication methods make use of an analog-to-digital(A/D) converter for transforming samples of analog signals into digitalformat for transmission. If multiple analog signals are to betransmitted, an analog signal multiplexer is employed to select onesignal at a time to feed to the A/D converter. A programmer or atelemetry system controller selects the channel from which the nextsample is to be converted prior to transmission. However, suchprocessing reduces the sampling rate per signal due to the relativelylarge portion of time and telemetry channel bandwidth which must be usedfor communicating the channel information. Moreover, a relatively fastA/D converter is required because the telemetry system must wait for theconversion to be completed before being able to transmit the data. Theuse of a fast A/D converter results in considerable energy usage,reducing the effective life of the implantable power source.

Alternatively, a predetermined data acquisition sequence may beestablished to eliminate the need for continuously communicating thechannel to be converted. This alternative, however, limits theflexibility of the system as the number and identity of channels to betransmitted generally cannot be changed without first reconfiguring thesequencer. Moreover, this technique requires the sampling process to besynchronized with read operations executed by the telemetry circuit, asasynchronous operation may yield transmission or reception of invalid ormisinterpreted data.

There is a need, therefore, for an improved technique for exchangingdata between an implantable device and a device external to a patient'sbody. There is a particular need for a telemetry technique which iscapable of transmitting digitized data to and from an implantabledevice, but which avoids certain of the drawbacks of existing systems assummarized above.

SUMMARY OF THE INVENTION

The present invention provides a novel technique designed to respond tothese needs. The technique permits the exchange of information betweenan implantable device and an external device, and the conversion ofanalog information to digital information according to and at ratesadapted to conform to the needs and desires of a user of the externaldevice, typically an attending physician. The telemetry techniqueenables the effective transmission of analog signals, such asintracardiac electrograms, intracardiac and spacial impedance signalsfrom the implantable device to an external device via high speed digitaltelemetry. In an advantageous configuration, the technique employsdedicated registers in the implantable device for storing datacorresponding to digitized values of analog signals associated with theregisters. The contents of the registers may be telemetered to theexternal device upon demand. In a preferred arrangement, the contents ofthe registers are updated automatically each time the register is read,refreshing the stored data contained in the register as a function ofthe read requests received from the external device. The A/D conversionprocess, its sequence and its speed are advantageously determined by therequests of the external device in real time, providing enhancedflexibility and reduced energy consumption, while offering the attendingphysician the most up-to-date information on the specific informationdesired to be accessed.

Thus, in accordance with a first aspect of the invention, a datatelemetry system is provided for transmitting signals from animplantable device to a remote external device. The implantable deviceis configured to collect data representative of at least first andsecond operational parameters of the implantable device or a biologicalsystem in which the implantable device is disposed. The telemetry systemincludes first and second memory circuits, a telemetry circuit, and acontrol circuit. The memory circuits allow for storage of valuesrepresentative of the first and second parameters, respectively. Thetelemetry circuit is coupled to the first and second memory circuits,and is configured to transmit first and second signals representative ofthe first and second values. The signals transmitted by the telemetrycircuit are in response to transmission request signals from the remotedevice. The control circuit is coupled to the first and second memorycircuits and is configured to control replacement of the first andsecond values in the first and second memory circuits in response totransmission of the respective first and second signals. Ananalog-to-digital conversion circuit is advantageously coupled to thefirst and second memory circuits and converts analog signals to thefirst and second values in response to transmission of the correspondingvalue via the telemetry circuit. A switching circuit may be provided forapplying analog signals to the conversion circuit as the first andsecond values are telemetered.

In accordance with another aspect of the invention, an implantabledevice is provided which is configured to be disposed in a living body.The device includes a signal processing circuit, a signal conversioncircuit, memory circuits, a telemetry circuit, and a control circuit.The signal processing circuit detects at least two operationalparameters of the device or the body, and generates analog parametersignals representative thereof. The signal conversion circuit is coupledto the signal processing circuit for converting the analog parametersignals to digitized parameter values. The memory circuits store thedigitized parameter values produced by the conversion circuit. Thetelemetry circuit transmits signals representative of the digitizedparameter values in response to request signals received from anexternal unit. The control circuit is coupled to the signal processingcircuit and is configured to apply analog parameter signals to theconversion circuit in response to transmission of the digitized values.The control circuit may advantageously control the conductive state ofswitches in a switching circuit for selectively applying the analogparameter signals to the conversion circuit in coordination with thetelemetry of the digitized values.

In accordance with still another aspect of the invention, a system isprovided for telemetering digital data from an implantable medicaldevice to an external device. The system includes a data acquisitioncircuit, an analog-to-digital converter, a telemetry circuit, and acontrol circuit. The data acquisition circuit is configured to generateanalog parameter signals representative of operational parameters of theimplantable device or a body in which the implantable device isdisposed. The analog-to-digital converter is coupled to the dataacquisition circuit for converting the analog signals to digital values.The telemetry circuit transmits digital values produced by the converterto the external device in response to request signals from the externaldevice. The control circuit selectively applies the analog signals tothe converter. The digital values are thus telemetered to the externaldevice in a sequence and at a rate defined by the request signals inreal time.

The invention also provides a method for transmitting data between animplantable device configured to be disposed in a living body and anexternal device disposed outside the body. In accordance with themethod, first and second analog parameter signals are generated whichare representative of operational parameters of the body or of theimplantable device. The analog parameter signals are converted todigital values, and the digital values are stored in a memory circuit.One of the digital parameter values is telemetered to the externaldevice in response to a request signal from the external device. Theanalog parameter signal corresponding to the telemetered parameter valueis then converted to an updated digital value. The telemetered parametervalue is then replaced in the memory circuit with the updated digitalvalue.

In accordance with a further aspect of the invention, a method isprovided for acquiring data representative of cardiac function. Themethod includes the steps of monitoring a plurality of parametersrepresentative of cardiac function in an implantable device, andgenerating analog parameter signals representative thereof. The analogparameter signals are converted to respective digital parameter values.The digital parameter values are stored in a memory circuit. A desireddigital parameter value is telemetered to an external device in responseto a request signal from the external device. The analog parameter valuecorresponding to the desired digital parameter value is then convertedto an updated digital value, and the desired digital parameter value isreplaced in the memory circuit with the updated digital value. Theanalog parameter signals may be derived from sensed signals, such as ina dedicated signal processing circuit. The method may be repeated toobtain effective sampling rates for the parameters as defined by therequest signals from the external device. Sampling rates may bedifferent for different parameters depending upon the particularparameter of interest and the rate of sampling required for obtainingmeaningful information on the parameter.

In accordance with still another aspect of the invention, a method isprovided for telemetering digital data from an implantable medicaldevice to an external device. According to the method, analog signalsare generated which are representative of operational parameters of theimplantable device or a body in which the implantable device is dispose.A series of data request signals are transmitted from an external deviceto the implantable device. The data request signals define a sequence ofdesired samples of the operational parameters. The analog signals areprocessed in the implantable device to convert analog signalscorresponding to the desired samples to digital values and to telemeterthe digital values to the external device in response to the datarequest signals. The data request signals may advantageously establisheffective sampling rates for specific parameters of interest, dependingupon the nature of the parameter, and the sampling rate required toobtain meaningful information on them.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome apparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is a diagrammatical representation of an implantable device, inthe form of a cardiac stimulator, coupled to a heart and arranged totelemeter parameter signals to an external unit;

FIG. 2 is a diagrammatical representation of certain functionalcircuitry of the implantable cardiac stimulator shown in FIG. 1,representing the interconnection between the circuitry and the flow ofsignals within the device;

FIG. 3 is a diagrammatical representation of certain signal acquisitionand signal processing circuitry for detecting and processing analogsignals representative of cardiac function in the device shown in FIG.2;

FIG. 4 is a diagrammatical representation of circuitry for receiving theanalog signals from the circuitry of FIG. 3, for converting the analogsignals to digital values, and for telemetering the digital values to anexternal unit; and

FIG. 5 is a flow chart representing exemplary control logic forprocessing analog signals in an implantable device of the type shown inthe previous Figures, so as to convert the analog signals to digitalvalues and to telemeter the values to an external unit.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Turning now to the drawings, and referring first to FIG. 1, animplantable device is illustrated diagrammatically and designatedgenerally by the reference numeral 10. The implantable device isconfigured for collecting data and for transmitting and receiving databased upon data requests. The implantable device exchanges the data withan external device, designated generally by the reference numeral 12. Inthe illustrated embodiment, implantable device 10 is a cardiacstimulator which is implanted in the body 14 of a patient in accordancewith generally known techniques. The cardiac stimulator collects andtransmits data regarding both its function and that of the body, andtransmits data upon request to external device 12 through tissues,represented diagrammatically at reference numeral 16. The advantageousmanner in which data is collected, processed and transmitted betweenimplantable device 10 and external device 12 is summarized in greaterdetail below.

Implantable device 10 includes data acquisition, processing andtelemetry circuitry housed within a case or enclosure 18. Enclosure 18is sealed to isolate the circuitry from surrounding tissues and bodyfluids following implantation. Leads 20 and 22 are interconnected withthe circuitry within enclosure 18 and extend to sensing points withinbody 14. In the case of the cardiac stimulator illustrated in theFigures, leads 20 and 22 are coupled to desired points within the heart24 of the patient. In particular, a terminal end 26 of lead 22 issecured within the right atrium 28 of the patient's heart, while aterminal end 30 of lead 20 is secured within the right ventricle 32 ofthe patient's heart. As will be appreciated by those skilled in the art,leads 20 and 22 are generally flexible assemblies including anelectrically conductive core surrounded by a protected sheath. Forexample, the internal core may be coiled titanium wire, and theprotective sheath may be a coating of polyurethane. Once secured withinthe respective portions of the patient's heart, tips 26 and 30 permitparameters representative of cardiac function to be sensed via signalstransmitted through the leads to circuitry within enclosure 18.Moreover, circuitry is provided within device 10 for stimulatingportions of the heart via leads 20 and 22 in a manner generally known inthe art.

It should be noted that, while throughout the present discussionreference is made to data acquisition, processing and telemetrytechniques as applied to a cardiac stimulator, the techniques may findapplication outside the realm of cardiac devices. In particular, signalsmonitored, processed and telemetered by implantable device 10 could berelated to the state and function of other biological systems withinbody 14, including organs and tissue other than the heart.

Once implanted within body 14, device 10 executes predeterminedmonitoring and control functions as required by the particular conditionof the patient. Signals monitored by device 10 may be accessed byexternal device 12 via an antenna 34. For monitoring, antenna 34 isplaced adjacent to the patient's body in the general vicinity ofenclosure 18. Antenna 34 is coupled via flexible conductors 36 to a baseunit 38, commonly referred to as a programmer. Programmer 38 permits auser, typically an attending physician, to access information sensed andprocessed by implantable device 10 via antenna 34 and conductors 36.

The functions executed by implantable device 10 are accomplished viaelectronic circuitry housed within enclosure 18. FIG. 2 is adiagrammatical representation of exemplary circuitry for carrying outthe processes of the cardiac stimulator of FIG. 1. As shown in FIG. 2,circuitry within enclosure 18 is coupled to heart 24 via leads 20 and22. In particular, lead 22 includes an atrial tip conductor 40 and anatrial ring conductor 42. Similarly, lead 20 includes a ventricular tipconductor 44 and a ventricular ring conductor 46. As will be appreciatedby those skilled in the art, the tip and ring conductors of leads 20 and22 supply signals for stimulating portions of heart 24, as well asconvey feedback or detected parameter signals to the circuitry frompoints at or adjacent to the ends of leads 20 and 22.

Referring now more particularly to the functional electronic circuitryillustrated in FIG. 2, signals transmitted along conductors 40, 42, 44and 46 are interfaced with electronic circuitry as follows. Conductors40 and 42 of lead 22 are coupled to an atrial stimulus generator 48.Similarly, conductors 44 and 46 of lead 20 are coupled to a ventricularstimulus generator 50. Atrial and ventricular stimulus generators 48 and50 are configured to transmit electrical pulses for stimulating tissueswithin the heart 24, in a manner generally known in the art. Moreover,signals transmitted along conductors 40, 42, 44 and 46 are tapped andapplied to a data acquisition and processing circuit 52. As describedmore fully below, circuit 52 is configured to analyze signalsrepresentative of the function of heart 24 (and more generally of body14), as well as of implantable device 10. In the illustrated embodiment,data acquisition and processing circuit 52 is particularly suited formonitoring fast-changing analog waveforms by means of comparison,filtering and amplification circuitry.

Signals processed by circuit 52 are available for further processing andtelemetry to external device 12 as follows. Signals of interestprocessed by circuit 52 are applied to a switching circuit 54. Switchingcircuit 54 includes a plurality of solid state switching devices, theconductive state of which is controlled by a telemetry andanalog-to-digital (A/D) logic circuit 56. Circuit 56 configuresswitching circuit 54 to open and close switches within circuit 54 toapply desired signals from data acquisition and processing circuit 52 toan A/D conversion circuit 58. Analog signals from circuit 52 applied toA/D conversion circuit 58 are converted to corresponding digital values.These corresponding digital values are stored in a memory circuit 60. Asdescribed below, the advantageous configuration of circuit 52, 54, 56,58 and 60 permits specific analog signals of interest to be convertedfrom monitored analog waveforms to digital values so as to update memorylocations within circuit 60 in response to information requests receivedfrom external device 12.

Digitized values stored within memory circuit 60 are available fortransmission to external device 12 via a telemetry circuit 62. Telemetrycircuit 62 receives command signals from a microprocessor 64. Inaddition to commanding operation of telemetry circuit 62, microprocessor64 receives and processes various signals from other functionalcircuitry related to the continuous monitoring and stimulating functionsof implantable device 10. In particular, in the illustrated embodimentmicroprocessor 64 is coupled to an activity sensor 66, aventricular-to-atrial interval timer circuit 68, anatrial-to-ventricular interval timer circuit 70, a memory circuit 72,and a general purpose A/D circuit 74. The configuration and operation ofcircuits 66 through 74 are generally known in the art. For example,activity sensor 66 may include an accelerometer which detects movementof the patient in which implantable device 10 is disposed. Signals fromactivity sensor 66 are typically used by microprocessor 64 as arate-responsive input, allowing microprocessor 64 to adapt itsmonitoring and stimulation functions in accordance with the patient'schanging activity level. Timer circuits 68 and 70 receive clock pulsesfrom microprocessor 64 and serve to count or indicate intervals betweencontractions of tissues within heart 24. Memory circuit 72 serves tostore the control routine executed by microprocessor 64, as well as dataacquired by or processed by microprocessor 64. In particular, memorycircuit 72 may store diagnostic and programming data which may bepreconfigured prior to implantation of device 10, or which may beconveyed to device 10 by telemetry after implantation. Finally, generalpurpose A/D circuit 74 permits microprocessor 64 to obtain digitalvalues of various functional parameters in the execution of itsmonitoring and control routines. In general, A/D circuit 74 is employedfor more slowly changing parameter values, such as lead impedance,battery condition, and so forth.

It should be noted that implantable device 10 may include circuitrywhich is different from or complimentary to the circuitry illustrated inFIG. 2 and described above. In particular, as will be appreciated bythose skilled in the art, device 10 includes a power supply (not shown)which furnishes a continuous source of electrical energy needed foroperation of the functional circuitry. The power supply will typicallyinclude a power storage battery, such as a lithium iodide or lithiumcarbon monofloride battery. Where desired, the power supply circuitrymay also include a voltage regulator for converting the voltage from thebattery to a desired level as required by the functional circuitry.

In operation, implantable device 10 is configured to detect analogwaveforms via leads 20 and 22 on a continual basis. The analog waveformsare applied to signal acquisition and processing circuit 52, andtherethrough to switching circuit 54. Certain of the signals processedby circuit 52 may be applied directly to microprocessor 64. For example,in the illustrated embodiment, single-bit digitized signals are appliedto microprocessor 64 directly from circuit 52 to indicate to themicroprocessor that a chamber signal (typically corresponding to atissue contraction), has been detected. Microprocessor 64 functions tomonitor these signals as well as signals from circuits 66, 68, 70, 72and 74. As a function of these signals and of the routine stored withinmemory circuit 72, microprocessor 64 triggers atrial and ventricularstimulus generator circuits 48 and 50 to provide pulses to heart 24 asneeded to regulate its function.

As mentioned above, circuitry within implantable device 10 permitssignals monitored by the device to be accessed and telemetered toexternal device 12 upon demand. FIG. 3 represents in greater detailcertain of the circuitry comprising signal acquisition circuit 52 andswitching circuit 54 which facilitates this feature of the device. Inparticular, in the embodiment illustrated in FIG. 3, signal acquisitionand processing circuit 52 includes an atrial sense amplifier 76, aventricular sense amplifier 78, an atrial ring-to-can intracardiacelectrogram (IEGM) amplifier 80, a ventricular ring-to-can IEGMamplifier 82, and an impedance sensor 84. Sense amplifier circuits 76and 78 process signals transmitted over the conductors of leads 20 and22 to obtain filtered waveforms defined by the signals. IEGM amplifiers80 and 82 are coupled to conductors of leads 20 and 22, respectively,which conduct signals from ring electrodes of the lead tips. Thesecircuits are also coupled to enclosure 18, as indicated by the groundpotential symbol in FIG. 3. Circuits 80 and 82 reference the signalsthey receive to the potential of enclosure 18, filter the resultingsignals and amplify them for further processing. Impedance sensor 84detects waveforms representative of heart impedance, in a mannergenerally known in the art.

Signals produced by circuits 76, 78, 80, 82 and 84 are further processedin signal acquisition and processing circuit 52, and made available forconversion to digitized values. In particular, signals from atrial senseamplifier 76 and ventricular sense amplifier 78 are applied to a switchbank 86 which includes a plurality of solid state switching devices. Theswitching devices of switch bank 86 may be opened and closed to applysignals available from circuits 76 and 78 to programmable gainamplifiers 88 and 90. As described more fully below, the conductivestates of the switches of switch bank 86, and the gains of amplifiers 88and 90 are defined by microprocessor 64 (see FIG. 2).

Signals produced by IEGM amplifiers 80 and 82 are further processed byan adder circuit 92. In particular, the signal output by IEGM amplifier80 is applied to one input node of adder 92, while the signal output byIEGM amplifier 82 is applied to a pair of input node switches 94.Switches 94 may be opened and closed under the direction ofmicroprocessor 64 (see FIG. 2) to selectively add or subtract thesignals from circuits 80 and 82.

The signals produced and processed by the foregoing circuitry areavailable for conversion to digitized values via output conductors 96,98, 100, 102, 104 and 106. In the illustrated embodiment, first andsecond output conductors 96 and 98 are coupled to programmable gainamplifiers 88 and 90, respectively. A third output conductor 100 iscoupled to directly to the output of IEGM amplifier 80. A fourth outputconductor 102 is coupled to the output node of adder 92. A fifth outputconductor 104 is coupled directly to the output of IEGM amplifier 82.Finally, a sixth output conductor 106 carries the signal produced byimpedance sensor 84.

Referring now to the specific circuit configurations illustrated in FIG.3, atrial and ventricular sense amplifiers 76 and 78 include subcircuitsfor comparing, filtering and amplifying the signals they monitor. Eachcircuit thus includes an operational amplifier 108 which comparessignals transmitted via respective tip and ring conductors within leads20 and 22. The output signals produced by operational amplifiers 108 areapplied to respective low pass filters 110. The signals output by lowpass filters 110 are conveyed to first sets of switches within switchingbank 86. The signal are further processed by respective second low passfilters 112, and third low pass filters 114. The signals output by lowpass filters 114 are conveyed to additional sets of switches withinswitch bank 86. The signals from low pass filters 114 are also furtherfiltered by high pass filters 116. The output signals from high passfilters 116 are applied to third sets of switches within switch bank 86.As will be appreciated by those skilled in the art, low pass filters110, 112 and 114, and high pass filters 116 serve to limit the bandwidthof the analog signal output by operational amplifiers 108.

In addition to the filtering circuitry described above, each senseamplifier 76 and 78 includes an evoked potential detector 118. Theevoked potential detectors receive the raw output from operationalamplifiers 108 and produce signals available via additional switcheswithin switch bank 86. As will be appreciated by those skilled in theart, evoked potential detectors 118 are configured to detect signalswithin a very short time delay after a pacing pulse from stimulusgenerating circuits 48 and 50. Outputs of the evoked potential detectorsserve to indicate whether such pacing pulses were able to capture thechambers of the heart stimulated by the pulses. Thus, the evokedpotential detectors amplify the signals produced by operationalamplifiers 108 very rapidly after pacing pulses, filtering the signal toverify chamber capture. Such verification may be used to regulatefurther stimulation pulses and thereby to avoid unnecessarily drainingthe implanted power source.

Circuits 76 and 78 also include programmable threshold-crossingcomparators 120 which produce pulses if the outputs from high passfilters 116 exceed predetermined thresholds. As will be appreciated bythose skilled in the art, signals produced by comparators 120 serve toindicate whether a chamber signal has been detected. Signals fromcomparators 120 are applied directly to microprocessor 64.

IEGM amplifiers 80 and 82 also include filtering and amplificationcircuitry as shown in FIG. 3. Each amplifier circuit thus includes anoperational amplifier 122 which is coupled to the ring anode conductorwithin leads 20 and 22, and to enclosure 18. Signals output byoperational amplifiers 122 are filtered through low pass filters 124 andhigh pass filters 126. Output signals from high pass filters 126 areamplified in programmable gain amplifiers 128. The signals are thenoutput to third and fifth output conductors 100 and 104, and are appliedto adder 92 as summarized above.

In the illustrated embodiment, certain of the settings used by thecircuitry of FIG. 3 are controlled directly by microprocessor 64. Forexample, gains employed by programmable gain amplifiers 88, 90,120 and128 are set by microprocessor 64. As will be appreciated by thoseskilled in the art, amplifiers 88, 90 and 128 serve to scale the signalsapplied to them so as to obtain amplitudes corresponding to the dynamicrange of A/D converter circuit 58. Moreover, the conductive states ofswitches within switch bank 86 and of switches 94 of adder 92 areconfigured by microprocessor 64. As described more fully below, thestates of the switches are preferably set in accordance with anacquisition configuration defined via external device 12 and conveyed todevice 10 by telemetry.

Signals produced and processed by the circuitry of FIG. 3 are madeavailable for conversion to digitized values, and for telemetry toexternal device 12. FIG. 4 represents an exemplary configuration ofcircuitry for executing such functions. As shown in FIG. 4, outputconductors 96, 98, 100, 102, 104 and 106 are coupled to a series ofselection switches, indicated generally by the reference numeral 130.Selection switches 130 are solid state switches which may be closed toapply signals from the output conductors to an A/D input line 132. Theconductive states of switches 130 are commanded by logic circuit 56 viacontrol signal conductors 134. A/D input line 132 delivers signalsapplied to it by closure of switches 130 to A/D conversion circuit 58.Circuit 58 thereafter converts the analog signal applied via the inputline 132 to a digital value under the command of logic circuit 56.Control signals for commanding operation of circuit 58 are applied bylogic circuit 56 via a control conductor 136. In addition to commandingconversion of the analog signals to digital values, logic circuit 56also preferably places A/D conversion circuit 58 in a sleep mode when noanalog signals are to be converted, and wakes circuit 58 from the sleepmode as required for conversion of the analog signals to digital values.

Digital signals produced by A/D conversion circuit 58 are stored in aseries of registers within memory circuit 60. In particular, the digitalvalues are output from A/D conversion circuit 58 via an A/D output bus138. Bus 138 is coupled to a series of registers 140 (denoted REG 1-REG6 in FIG. 4) which are dedicated to the signals applied to circuit 58via analog output lines 96-106. Registers 140 store the digitized valuesand hold the values available for output to telemetry circuit 62 via adigital output bus 142. In the illustrated embodiment, registers 140 are8-bit registers which are appropriately addressed by cooperation oflogic circuit 56 and conversion circuit 58.

As mentioned above, external device or programmer 12 is configured topermit a programmer, typically an attending physician, to accessinformation stored in registers 140. As illustrated diagrammatically inFIG. 4, external device 12 generally includes antenna 34, a telemetryinterface circuit 144, a control circuit 146, and a human interface 148.Telemetry interface circuit 144 is configured to encode or encryptsignals, particularly data request signals, from external device 12which are transmitted to telemetry circuit 62 via antenna 34. Interfacecircuit 144 is also configured to receive data signals from telemetrycircuit 62 via antenna 34 and to decrypt the data signals. Controlcircuit 146 may execute a variety of signal processing and controlfunctions as desired by the particular application. For the presentpurposes, control circuit 146 serves to receive programmer inputs fromhuman interface 148 and to prompt interface circuit 144 to send requestsignals for data from device 10. Control circuit 146 is furtherconfigured to translate received data from device 10 to a useable form,and to output or display the data via human interface 148.

Telemetry circuit 62 and telemetry interface circuit 144 are preferablyconfigured to exchange data via magnetic fields which extend throughtissue 16 partially surrounding device 10. While various signaltransmission protocols may be envisioned and employed for implementingthe present data acquisition and telemetry technique, a presentlypreferred method is disclosed in U.S. Pat. No. 5,383,912 issued on Jan.24, 1995 to Cox et al., and U.S. Pat. No. 5,480,415 issued on Jan. 2,1996, also to Cox et al. Both of the foregoing patents are assigned tothe assignee of the present invention and are hereby incorporated intothe present disclosure by reference.

Digitized parameter values stored within memory circuit 60 aretelemetered to external device 12 in response to request or promptsignals received from the external device. FIG. 5 represents steps inexemplary control logic for accessing information stored in memorycircuit 60, for telemetering the information in response to requestsignals, and for refreshing the telemetered information in real time. Asmentioned above, prior to executing the telemetry and data conversionand storage steps summarized in FIG. 5, an attending physician or otheroperator will generally store an acquisition configuration for aparticular experiment or set of data readings of interest. Inparticular, in the illustrated embodiment, the attending physician willtransmit signals via the telemetry circuit to set switches of the switchbank 86 and switches 94 upstream of adder 92 (see FIG. 3), as well asdesired gains of the programmable gain amplifiers described above. Forexample, switches of switch bank 86 may be opened and closed to providebroad or narrow band IEGM data or EPD data along first and second outputlines 96 and 98. Based upon the instruction set provided tomicroprocessor 64 to establish this acquisition configuration, theconductive states of the switches in switch bank 86 and of switches 94,and the gains of the circuit amplifiers are set by microprocessor 64.

With the acquisition configuration thus set, external device 12telemeters data request signals as desired by the attending physician.The request signals are received by telemetry circuit 62, promptingtelemetry circuit 62 to access the requested values stored in memoryregisters (see registers 1 through 6 in FIG. 4) and to telemeter signalsrepresentative of the values to the external device. As data is accessedfrom each register of memory circuit 60, a logical flag is set to informtelemetry and A/D logic circuit 56 (see FIG. 2) that the register hasbeen read and its contents have been telemetered. Circuit 56 resetsswitches 30 so as to feed the analog signal corresponding to theaccessed register to the A/D conversion circuit 58. Thus, telemetry andA/D logic circuit 56, in cooperation with switching circuit 54 and A/Dconversion circuit 58, updates or refreshes the digitized values readfrom memory circuit 60 each time the values are accessed andtelemetered.

This control logic, designated generally by reference numeral 150, issummarized in FIG. 5. As indicated in FIG. 5, at step 152 a read requestsignal is transmitted by external device 12 and is received by telemetrycircuit 62. The request signal identifies a particular digitized valuewhich is desired to be telemetered, or a corresponding register in whichthe digitized value is stored (represented by the “i” in thenomenclature of FIG. 5). At step 154 the requested digitized parametervalue is accessed and telemetered via telemetry circuit 62. As mentionedabove, a logical flag is then set, indicating that register i has beenaccessed. At step 56, telemetry and A/D logic circuit 56 checks for suchlogical flags to determine whether a register has been accessed andread. When the circuit identifies that a particular register has beenread, the logic advances to step 158. When the outcome of step 156 isnegative, the logic returns to the upstream side of step 156 to continueto monitor for flags indicating that the registers have been read.

At step 158 circuit 56 verifies whether A/D conversion circuit 58 isavailable for converting an additional analog signal to a digital value.The circuit logic continues to loop back through this inquiry until A/Dconversion circuit 58 becomes available. Once the circuit is available,logic circuit 56 changes the conductive states of selection switches 130(see FIG. 4) to apply the analog signal corresponding to the digitalvalue read from register i to conversion circuit 58. With the switchesthus set, the then-current amplitude of the corresponding analog signalis applied to conversion circuit 58. At step 162, the conversion isperformed, generating a digitized value corresponding to the analogsignal. At step 164 the new digitized value is addressed and stored inthe corresponding register i from which the digitized value was accessedand telemetered, thereby refreshing the register with updatedinformation. As indicated at step 166, once the register i has beenrefreshed, the system will enter an idle mode. In this idle mode, logiccircuit 56 will again monitor logical flags for each register, awaitingan indication that a register has been accessed and its digital valuetelemetered. Upon detecting such a flag, circuit 56 will again executesteps 52 through 64 for the newly accessed register value.

As will appreciated by those skilled in the art, the foregoing techniqueoffers a number of significant advantages over existing data acquisitionand telemetry schemes. For example, logic circuit 56 will apply analogsignals to AND conversion circuit 58 in a manner and sequence conformingto the specific needs and requests of the external device. Moreover,only the parameters of interest to the attending physician will beconverted and telemetered, thereby reducing the power consumed by thedevice and more efficiently utilizing the signal processing capabilitiesand telemetry bandwidth.

By way of example, an attending physician may set an acquisitionconfiguration to obtain filtered IEGM data and impedance sense dataonly. In the acquisition configuration, then, switches in switch bank 86(see FIG. 3) would be set to transmit signals from points downstream ofhigh pass filters 116. External device 12 then sends request signals ina string for accessing the contents of registers corresponding to analogoutput lines 96, 98 and 106. The digital values are accessed andtelemetered in response to the request signals. As the data is accessedand telemetered, the corresponding analog signals are, in turn,sequentially converted to digital values; the digital values are storedin memory circuit 60; and the telemetry and memory updating routinesummarized in FIG. 5 is carried out for each requested data value. Solong as request signals are received from the external device and therequested data is accessed and sent, corresponding analog signals areaccessed and converted to digital values used to refresh the accessedmemory registers. Once logic circuit 56 detects that no request orprompt signal has been received for a predetermined time period, it putsA/D conversion circuit 58 in a sleep mode, thereby further reducingenergy consumption.

Another advantage afforded by the present technique is the ability toset and change the sequence and sampling rate for particular parametersin real time by the request signals received from the external device.In particular, because meaningful information regarding particularparameters may require different sampling rates, external device 12 maybe configured to request digitized values of changing analog waveformsin different sampling frequencies depending upon the particularparameter. Moreover, certain of the sensed parameters may change atspeeds permitting relatively slow sampling rates, while other parametersrequire extremely high sampling rates to obtain meaningful information.For example, data request signals prompting telemetry of digital valuescorresponding to the output of impedance sensor 84 may be sent at afrequency resulting in a sampling rate on the order of 100 to 150samples per second. Filtered IEGM data may be requested and telemeteredat a higher rate, such as of on the order of 200 to 300 samples persecond. Other IEGM data may be sampled on the order of 400 to 500samples per second. Very wide band IEGM data may require much highersampling rates, such as on the order of 1000 to 3000 samples per second.

Such different sampling rates can be readily accommodated by the presenttechnique. Request signals are assimilated into an appropriate stringdefining both the parameters to be converted and telemetered, as well asthe resulting sampling rate. By changing the request signal string, theattending physician may, in real time, access different information oralter the effective sampling rate of the accessed information. Toaccommodate very high sampling rates, A/D conversion circuit 58 ispreferably selected so as to permit the necessary conversion of anyparticular analog signal to a digitized value at least as fast as thehighest anticipated sampling rate. That is, A/D conversion circuit 58 isconveniently selected to provide an A/D conversion time approximatelyequal to or faster than the telemetry time anticipated.

As noted above, while the foregoing technique has been described in thecontext of a cardiac stimulator, in appropriate devices, it may beemployed for providing information relating to other tissues and organs.Moreover, parameters in addition to those described above may beaccessed, processed and telemetered in accordance with the foregoingtechnique. In particular, in a cardiac stimulator, other possible signalsources include implanted pressure sensors, such as transducersconfigured to generate signals indicative of intracardiac pressure.Moreover, signals may be processed from such sources as peak endocardialaccelerometers, for providing an indication of global contractility ofthe heart as a function of a signal amplitude.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A data telemetry system, the telemetry systemcomprising: an implantable device, wherein the implantable device isconfigured to collect data representative of at least first and secondoperational parameters of the implantable device or a biological systemin which the implantable device is disposed; a remote external device,wherein the remote external device produces transmission signals toprompt the implantable device to transmit signals to the remote externaldevice; first and second memory circuits for storing first and seconddigitized values representative of the at least first and secondoperational parameters, respectively; a telemetry circuit coupled to thefirst and second memory circuits and configured to transmit the firstand second digitized values stored in the first and second memorycircuits, respectively, in response to the transmission prompt signalsfrom the remote device; and a control circuit coupled to the first andsecond memory circuits to provide control signals thereto, the controlcircuit being configured to control collecting data representative ofthe first operational parameter and updating the first digitized valuein the first memory circuit in response to transmission of the firstdigitized value, and collecting data representative of the secondoperational parameter and updating the second digitized value in thesecond memory circuit in response to transmission of the seconddigitized value.
 2. The telemetry system of claim 1, further comprisinga conversion circuit coupled to the first and second memory circuits andthe control circuit, wherein a transmission of the digitized valuecontained in first memory circuit prompts the conversion circuit toconvert the data representative of the first operational parameter to afirst digitized value and store the first value in the first memorycircuits, and a transmission of the digitized value contained in thesecond memory circuit prompts the conversion circuit to convert the datarepresentative of the second representative of the second operationalparameter to a second digitized value and store the second digitizedvalue in the second memory circuit respectively, in response to thecontrol signals from the control circuit.
 3. The telemetry system ofclaim 2, further comprising a switching circuit coupled to theconversion circuit and to the control circuit, the switching circuitbeing operative to apply the data representative of at least first andsecond operational parameters to the conversion circuit for conversionto the first and second values in response to the control signals fromthe control circuit.
 4. The telemetry system of claim 3, wherein theimplantable device includes signal processing circuitry for deriving theanalog signals from sampled signals, and wherein the switching circuitis coupled to the signal processing circuitry.
 5. The telemetry systemof claim 1, wherein the implantable device includes a cardiac stimulatorand wherein the operational parameters include at least onephysiological parameter representative of a state of a heart.
 6. Animplantable device configured to be disposed in a living body, thedevice comprising: a signal processing circuit for detecting at leasttwo operational parameters of the device or the body and for generatingfirst and second analog parameter signals representative thereof; asignal conversion circuit coupled to the signal processing circuit forconverting the first and second analog parameter signals to first andsecond digitized parameter values, respectively; first and second memorycircuits coupled to the conversion circuit for storing the first andsecond digitized parameter values, respectively; a telemetry circuitcoupled to the memory circuits for transmitting signals representativeof the first and the second digitized parameter values in response tofirst and second transmit signals, respectively, from an external unit;and a control circuit coupled to the signal processing circuit andconfigured to apply the first analog parameter signal to the conversioncircuit in response to the first transmit signal and to apply the secondanalog parameter signal to the conversion circuit in response to thesecond transmit signal.
 7. The implantable device of claim 6, whereinthe control circuit includes at least first and second switchesconfigured to change conductive states to apply the first and secondanalog parameter signals to the conversion circuit.
 8. The implantabledevice of claim 7, wherein the control circuit includes a logic circuitconfigured to command the conductive state of the first and secondswitches in response to the first and second transmit signals.
 9. Theimplantable device of claim 6, further comprising at least one leadcoupled to the signal processing circuit and configured for coupling totissue of the body.
 10. The implantable device of claim 9, wherein theat least one lead is adapted to be coupled to a heart.
 11. Theimplantable device of claim 6, wherein the control circuit includes ananalog data bus coupled to the conversion circuit, and wherein thecontrol circuit applies the first and second analog parameter signals tothe conversion circuit via the analog data bus.
 12. The implantabledevice of claim 6, further comprising an input digital data bus coupledto the conversion circuit and to the first and second memory circuitsfor conveying the first and second digital parameter values from theconversion circuit.
 13. The implantable device of claim 6, furthercomprising an output digital data bus coupled to the first and secondmemory circuits and to the telemetry circuit for applying the first andthe second digital parameter values to the telemetry circuit.
 14. Theimplantable device of claim 6, wherein the signal processing circuit isconfigured to derive the analog parameter signals from detected samplesignals.